1. Field
Embodiments described herein relate generally to a semiconductor memory device and a writing method thereof.
2. Description of the Related Art
There is a rapid increase in demand for NAND type flash memory along with increase of applications of mobile devices that process large-size data such as images, motion pictures, etc. Especially, by employing a multi-value storage technique that enables information of two bits or more to be stored in one memory cell, NAND type flash memory has become able to store more information with a smaller chip area. NAND type flash memory uses, for example, a floating-gate type memory cell and stores data in the memory cell by storing charges in the floating gate and thereby changing the threshold voltage of the memory cell.
However, along with the progress of miniaturization, it is becoming difficult to maintain data reliability by using this data storing scheme based on storing of charges in the floating gate. That is, in a highly-integrated flash memory with a high degree of miniaturization of cells, threshold voltage distributions fluctuate due to interference between adjoining cells, raising a problem that data reliability is damaged. Although various techniques have been proposed to prevent fluctuation of threshold voltage distributions, a fundamental solution has not been achieved yet. Hence, there is a demand for memory cells that do not let the stored data fluctuate due to influence from adjoining memory cells, even when miniaturization progresses.